One of the mainstream approaches employed in recent washing machines is a field oriented control or a vector control of the motor to rotate the drum by direct drive method. The above described configuration provides improved rotation accuracy which in turn yields improved wash performance as well as reduced vibration and noise during the operation of the washing machine. Under the above described configuration, when the drum is rotated at high speed, for instance, for dehydration, d-axis current which does not contribute to torque output is energized to execute a field weakening control that reduces induced voltage occurring at the stator winding. One of the problems encountered in field weakening control is increased copper loss caused by energization of d-axis current, which inevitably reduces the motor drive efficiency.
One of the approaches for addressing such problem is disclosed for example in JP 2008-266386 A in which permanent magnets having low coercivity are disposed on the rotor side of a 48 pole 36 slot configuration motor and the permanent magnet is demagnetized by momentarily energizing the stator winding with large current. The reduction in magnetic flux of the permanent magnet reduces the induced voltage produced at the motor to allow high speed operation without executing a field weakening control. One example of a permanent magnet motor employing the above described structure is disclosed in JP 2006-280195 A.
However, it has been found that the attempt to modify the amount of magnetization in the above described manner caused magnetization/demagnetization of the permanent magnet from time to time depending upon the structure of the motor even when q-axis current is energized for the purpose of generating torque. FIG. 10 indicates the measurement of magnetization. First, the permanent magnet is magnetized to execute a wash operation with a washing machine such that 43 volts of induced voltage is generated. When wash operation is executed under such state and winding current is energized to approximately 10 A during the wash operation, demagnetization was observed in which induced voltage generated by the motor was reduced to 38V. The induced voltage showed a variance ranging from 34V to 42V in the range of ±10 A as indicated in FIG. 10.
To elaborate on the above findings, when current of approximately 10 A is energized when outputting torque, current for magnetizing and demagnetizing the permanent magnet in equal amounts is flown in the level of ±10 A which causes the permanent magnet in magnetized state to be demagnetized to approximately 34V of induced voltage, and the permanent magnet in demagnetized state to be magnetized to approximately 42V of induced voltage. Because this happens alternately when q-axis voltage is energized, magnetization is consequently stabilized at an intermediate voltage of approximately 38V as described earlier.
Because the wash operation of the washing machine requires high torque output, generation of relatively greater amount of induced voltage is desirable. However, when permanent magnet is demagnetized by the energization of q-axis current, greater amount of q-axis current needs to be energized to obtain the desired torque, which consequently increases electricity consumption.